Various interfermetric measuring techniques and devices have been developed in the past and are still being used. With the advent of optical fibers, a large number of interferometric measuring systems employing fiber optics has been proposed. Such systems have widespread application in measuring vibration and dynamic distortion of mechanical components. Moreover, fiber optic interferometers are especially suited to metrology because of the flexibility and size of the components involved.
Fiber optic interferometers of the prior art include those characterized as modified Mach-Zehnder or Twyman-Green interferometers. In their simplest form, these interferometers use light from a coherent source split into two optical beams. One of these beams constitutes a reference beam and traverses an optical path of a fixed length. The other beam constitues a probe beam and is guided along an optical path whose length is altered by the movement of an external object or workpiece being measured. The reference and probe beams are subsequently coherently recombined to produce an interference pattern indicative of the vibration or dynamic distortion of the mechanical element.
Heterodyne optical interferometers are also well known in the art. These devices are similar to basic Mach-Zehnder and Twyman-Green interferometers but are modified to include an optical modulator which shifts the optical frequency of the reference and/or probe beam. As in the basic Mach-Zehnder or Twyman-Green interferometers, the optical path length of the probe beam is altered by the vibrating external object. The reference and probe beams are again subsequently recombined, yielding a frequency modulated (FM) beam with a carrier frequency equal to the frequency of the optical modulator and deviations from the carrier frequency caused by the vibration or dynamic distortion of the external object. The deviations which are caused by the motion of the external object are then extracted by conventional FM demodulation techniques.
In fiber optic measuring or gauging systems of the prior art, the reference and probe beams are usually guided along optical paths that include separate optical fibers. This configuration ensures that a truly stationary reference wavefront is used for comparison with the unknown Doppler shifted wavefront returning from the object. However, the optical fibers act as microphones in picking up environmental noise. This environmental noise signal is comprised of unwanted Doppler shifts due to environmental vibrations and attendant slight variations in the refractive index of the optical fiber carrying the probe or reference beam, and is added to the measured vibration signal from the object the motion of which is being measured, resulting in a distortion of the actual vibration frequency and amplitude signature imposed onto the returning light beam. This has, in effect, precluded the use of fiber optic conventional or heterodyne interferometers in typical manufacturing environments.
To avoid this drawback, it was proposed in a commonly owned U.S. Pat. No. 4,627,731, issued Dec. 9, 1986 and entitled "Common Optical Path Interferometric Gauge", to use a laser source that produces a laser beam having a relatively short coherence length and to combine the reference and probe beams, after the reference beam has been delayed relative to the probe beam by a time interval of such a length that the reference beam lags behind the probe beam by a distance at least equal to but advantagously exceeding the aforementioned coherence length, and let the thus combined beam propagate in a common path, especially in an optical fiber, toward a location that is spaced by substantially one-half of the above distance from the surface of the object the motion of which is to be measured. At the above location, a portion of the combined beam was reflected back into the common path, while the remainder of the combined beam was aimed at a predetermined zone of the object surface and returned therefrom back to the above location and into the common path, where the returned part of the probe beam, which has been influenced by the motion of the aforementioned zone, has coherently interfered with the portion of the reference beam part that has been reflected at the above location back into the common path.
While this approach has essentially eliminated the above problem, inasmuch as the environmental noise had influenced both the probe beam and the reference beam in substantially the same manner, resulting in cancellation of the influence of the environmental noise on the interference pattern, this type of interferometric arrangement still possessed a drawback in that it was dependent on the location of the above-mentioned location with respect to the speckle pattern which is formed as the remainder of the combined beam is returned back from the affected zone of the object surface. More particularly, the above zone has a finite area which exhibits surface irregularities having magnitudes capable of distorting the wavefront of the returning combined beam even when the object surface is of a very high surface quality, so that the remainder of the combined beam is scattered to a certain degree during its reflection from the above zone and forms a speckle pattern that includes bright and dark speckles due to interference in the returning combined beam. For obvious reasons, it is not always possible to assure that the above location is situated at the bright speckle; when it is not, there occurs a signal dropout due to the low or non-existent intensity of returning light at the above location due to its alignment with the dark speckle.
Accordingly, it is a general object of the present invention to avoid the disadvantages of the prior art.
More particularly, it is an object of the present invention to provide a method of measuring the movement of a vibrating object, which does not possess the disadvantages of the known methods of this kind.
Still another object of the present invention is so to develop the method of the type here under consideration as to virtually assure that reliable measurement results are obtained in spite of the existence of the speckle pattern.
A concomitant object of the present invention is to devise a measuring arrangement capable of performing the method of the above type.
An additional object of the present invention is to design the above measuring arrangement in such a manner as to present reliable measuring results regardless of the position of the aforementioned location relative to the speckle pattern.
It is yet another object of the present invention to construct the mirror arrangement of the above type in such a manner as to be relatively simple in construction, inexpensive to manufacture, easy to use, and yet reliable in operation.